1. Field of the Invention
The present invention relates to a speaker including a plurality of diaphragms and a mobile terminal device having such a speaker mounted thereon.
2. Description of the Related Art
A speaker including one magnetic circuit and two diaphragms is disclosed in, for example, Japanese Laid-Open Publication No. 62-277000, Japanese Laid-Open Utility Model Publication No. 4-135096, and Japanese Laid-Open Publication No. 11-252683. The speaker disclosed in Japanese Laid-Open Publication No. 11-252683 will be described below.
FIG. 22 is a cross-sectional view of a speaker 3000 disclosed in Japanese Laid-Open Publication No. 11-252683. The speaker 3000 includes a pole piece 1 having a recessed portion which has a hole 14 at a center thereof, a yoke 2 which is recessed at a center thereof and having a hole 13, and a ring-shaped magnet 3 fixed between the pole piece 1 and the yoke 2. The yoke 2 acts as a frame. The speaker 3000 has a first magnetic gap 4 on an outer side of the magnet 3 and a second magnetic gap 5 on an inner side of the magnet 3. The speaker 3000 further includes a first voice coil 6 inserted into the first magnetic gap 4, a second voice coil 7 inserted into the second magnetic gap 5, a first diaphragm 8 having an outer peripheral area supported by the yoke 2 and a central area connected to the first voice coil 6, a second diaphragm 9 having an outer peripheral area supported by the yoke 2 and having a central area connected to the second voice coil 7, a first protector 10 provided so as to face the first diaphragm 8, and a second protector 11 provided so as to face the second diaphragm 9. The speaker 3000 is mounted on a printed circuit board 12 of a device. The printed circuit board 12 has a hole 15 aligned with the hole 13.
An exemplary operation of the speaker 3000 will be described. When an electric signal is input to the first voice coil 6, the first diaphragm 8 vibrates so as to generate a sound. The sound is output through the hole 13 of the yoke 2 and the hole 15 of the printed circuit board 12. The holes 13 and 15 are formed so as not to disturb the movement of the sound wave. When an electric signal is input to the second voice coil 7, the second diaphragm 9 vibrates so as to generate a sound. The sound is output through the hole 14 of the pole piece 1, the hole 13 of the yoke 2, and the hole 15 of the printed circuit board 12. As well as the holes 13 and 15, the hole 14 is formed so as not to disturb the movement of the sound wave. In the state where the speaker 3000 is mounted on a cellular phone, the second diaphragm 9 is used as a receiver, and the first diaphragm 8 having a larger diameter than that of the second diaphragm 9 is used as a sounder for reproducing a calling sound.
Although not described in Japanese Laid-Open Publication No. 11-252683, the sound from the first and the second diaphragms 8 and 9 is usually directly output through holes formed in the first and second protectors 10 and 11.
Such a structure has the following problem. In a cellular phone, which is most popular among various mobile terminal devices, a liquid crystal display section provided on a front surface of a casing is now increasing in size in order to display a larger number of characters and a larger amount of image information. Such an increased size of the liquid crystal display section reduces a space available for a speaker for reproducing a calling sound which is adjacent to the liquid crystal display section. The speaker diameter was about 20 mm several years ago, but has now been reduced to about 10 mm or less. A simple reduction in size of the diaphragms or magnetic circuit made in accordance with the reduced space for the speaker results in problems, for example, in that a sufficient sound pressure level is not obtained and a sufficient volume of low frequency sound is not obtained. A speaker having a relatively large speaker diameter of 20 mm provides sufficient acoustic characteristics, but requires a large space to the side of the display section of the cellular phone due to the structure of the cellular phone of outputting the sound through the holes of the protectors. This conflicts with the increase in the size of the display section. The structure of outputting the sound through the holes of the protectors also limits the position from which the sound may be output to, for example, a portion above the display section.
The speaker 3000 further requires that an edge portion of the second diaphragm 9, which determines the minimum resonance frequency (i.e., a portion of the second diaphragm 9 between a position connected to the second voice coil 7 and a position connected to the yoke 2), be sufficiently large. Otherwise, the stiffness of the edge portion is too large to sufficiently reproduce a low frequency sound. In order to make the edge portion large, the magnet 3 needs to be reduced in size so that the diameter of the second magnetic gap 5 is reduced or the diameter of the second diaphragm 9 needs to be increased. The diameter of the first magnetic gap 4 also needs to be reduced, or the diameter of the first diaphragm 8 needs to be increased. However, a reduction in the size of the magnet 3 causes the problem in that the magnetic energy becomes insufficient, resulting in a reduction in the driving efficiency of the speaker 3000. An increase in the diameter of the second diaphragm 9 causes the problem of an increase in the speaker diameter of the speaker 3000, resulting in an inability to save space.
In the speaker 3000, a bottom surface of the first diaphragm 8 and a top surface of the second diaphragm 9 are acoustically coupled to each other through the hole 14 of the pole piece 1. Therefore, when an electric signal is applied to the first voice coil 6 so as to reproduce a sound by the first diaphragm 8, the sound generated by the bottom surface of the first diaphragm 8 changes the air pressure on the top surface of the second diaphragm 9 through the hole 14, thereby vibrating the second diaphragm 9. This causes the problem in that even when no electric signal is applied to the second voice coil 7, a sound may be generated by the second diaphragm 9. Likewise, when an electric signal is applied to the second voice coil 7 so as to reproduce a sound by the second diaphragm 9, the first diaphragm 8 may be driven by the second diaphragm 9 to generate a sound. Due to these problems, the first diaphragm 8 and the second diaphragm 9 cannot be independently driven.